1. Field of the Invention
The present invention relates to the field of video image display and, more specifically, to a video signal preamplifier intended to be interposed between a composite video signal source (RGB+synchronization signals) and a signal amplifier for a monitor. The preamplifier to which the present invention relates is intended for receiving analog signals.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional example of a video display signal including a preamplifier of the type to which the present invention relates. A video source 1 formed, for example, of a graphics board of a microcomputer, provides video signals for a display monitor. The signals provided by source 1 basically include a video signal including the three RGB color component data, synchronization signals, and a so-called xe2x80x9cblankingxe2x80x9d signal (BLK) for setting a reference level between two display windows. Before being able to be displayed on a monitor screen 2 while being exploited by the processing circuits of this monitor, the video signals must be submitted to an analog processing generally including, successively, a level shifting in a circuit 3 (CLAMP), a preamplification within a circuit 4 and finally an amplification in a circuit 5. The function of amplifier 5 only is to amplify the video signal at the end of the processing by circuits 3 and 4. The function of preamplifier 4 is, in addition to preamplifying the video signal (the gain of amplifier 5 being insufficient), to set the base level of the video signal. The function of clamp 3 is to set the voltage level of the video signal to enable it to drive the preamplifier. This setting is performed outside of the display windows, for example, during line and frame flybacks. Circuit 3 receives, for this purpose, line flyback and frame flyback synchronization signal Hsync. Preamplifier 4 receives a reference voltage level that depends on the reference of the video signal at the input of this preamplifier, as will be seen hereafter. It should be noted that the RGB video signals transit through a connection capacitor C to isolate source 1 from the rest of the monitor. Circuits 3, 4, and 5 belong to the conventional analog processings of a video monitor. Generally, the input impedance of the monitor is standardized and determined by a resistor (R) between a first electrode of capacitor C and the ground. It should be noted that signals other than the RGB signal are provided to the monitor.
The different analog processings of the monitor within circuits 3, 4, and 5 have as a main function, in addition to signal amplification, ensuring the alignment of the video signal and to analogically transmit the video. More specifically, preamplifier 4 has, in addition to the function of amplifying the signal due to the insufficient gain of amplifier 5, that of determining a so-called black level Vb of the video signal as well as a signal V0 of this signal during line flyback and frame flyback periods.
FIGS. 2A, 2B, and 2C illustrate, in the form of simplified timing diagrams, an example of operation of an analog processing circuit of a video monitor such as illustrated in FIG. 1. FIG. 2A shows an example of shape of an RGB signal formed of analog voltage levels during display windows between two line flybacks. FIG. 2B shows the shape of a blanking signal BLK including pulses between two display windows. In FIG. 2B, it is assumed that all the stages (pulses) of signal BLK occur during line flybacks (or frame flybacks) having generally longer durations. FIG. 2C illustrates an example of the shape of signal V out provided by preamplifier 4. Signal V out reproduces the analog levels of the RGB signal now aligned on black level Vb and mixed with the stages of signal BLK that are restored in a level V0.
Levels Vb and V0 are set within preamplifier 4 to set the levels of the constant monitor control information. In particular, as previously indicated, level Vb determines the black level of the monitor.
The operation of the monitor circuits being perfectly well known, it will not be further detailed. Only the elements that will subsequently enable better understanding an example of application of the present invention and its advantages have been illustrated and discussed.
FIG. 3 shows, still schematically and in the form of blocs, the portion of the analog processing circuit of FIG. 1 which is more specifically associated with the video preamplifier input stage. As illustrated in FIG. 3, a preamplifier (4, FIG. 1) is essentially formed of a differential stage 6 (DIFF) providing signal V out and receiving, as differential inputs, reference level Vref and a signal V out coming from an input stage 7 (IN) of the preamplifier. Input stage 7 especially has the function of adapting the level of the input signal to the input impedance of stage 6. Input stage 7 receives video signal RGB of clamp 3, the function of which is to place the level of the preamplifier input signal at voltage Vref when the video signal is low, that is, during the periods of horizontal synchronization pulses (signal Hsync), and thus to place the input at the reference level outside the display windows. As illustrated in FIG. 3, differential circuit 6 receives level data Vb, V0 as well as signal BLK, which must be combined with the RGB signal to form signal V out.
Input stage 7 of a video preamplifier is a portion of the signal which is particularly important. Indeed, it is the first link of an analog processing chain that must have good performance in terms of rapidity and linearity. Conventionally, the ROB signal must be aligned with respect to level Vref to have sufficient voltage drops in the preamplifier. A bipolar technology is generally used within clamp 3 and input stage 7. In practice, the input level is compared to level Vref to control the passing of a current through a bipolar transistor of circuit 3, that is, to recharge or discharge capacitor C during horizontal synchronization periods Hsync to set level Vref during these periods. A problem that is posed in conventional circuits using bipolar transistors is that the high current need in the preamplifier tends to shift the voltage level of capacitor C since the base current of the bipolar transistor driving the differential stage is not negligible. A solution would be to increase the gain of this transistor. However, an increase of the input transistor gain introduces a decrease of its switching speed.
Another problem of conventional circuits is that, in the case where there is no synchronization signal, the shaping switch used to discharge or charge capacitor C must be closed for the input signal to then be at the reference level, so that differential stage 6 detects no level difference with respect to its other input. The existence of a base current of the transistor causes a charge of the input capacitor, which progressively causes the transmission of a white information level to the display amplifier, while the monitor must remain black.
It would be desirable to be able to align the input RGB video signal of the differential stage with respect to ground. This would enable, among other things, accelerating the response of the preamplifier stage, to provide reduced power consumption. In a conventional bipolar transistor circuit, such a solution is not possible since the leakage resistor that would then be required to discharge the input capacitor would generate a permanent decrease of the level of this charge and, accordingly, a distortion of the white level of the displayed image.
The present invention aims at providing a novel video preamplifier that overcomes the disadvantages of known systems and, in particular, that enables aligning the input video signal with respect to ground.
The present invention also aims at providing a solution that maintains the rapidity and linearity of the preamplifier.
More generally, the present invention aims at providing a differential preamplifier including an input stage intended for receiving an analog signal through a connection or isolation capacitor.
To enable aligning the input signal with respect to ground, it is necessary to use a follower-mounted MOS transistor as the preamplifier input stage. The use of a MOS transistor also enables a null input current control, which is an unquestionable advantage in terms of charge hold of input capacitor C, which was a problem of conventional solutions.
Thus, a first feature of the present invention is to provide a video preamplifier including, as an input stage, a follower-mounted PMOS transistor.
More specifically, the present invention provides a preamplifier including an input stage adapted to receiving an analog signal via a connection capacitor, and a differential output stage adapted to providing said signal referenced with respect to a predetermined level, including means for enabling the input stage to accept a signal referenced to the differential stage ground, the signal provided by the output stage being referenced to this ground.
According to an embodiment of the present invention, the input stage includes a first MOS transistor, the gate of which receives said input signal referenced to ground, in series with a first current source, the midpoint between said current source and said transistor defining the output terminal of the preamplifier input stage.
According to an embodiment of the present invention, the preamplifier includes a switching means for drawing the output of the input stage to the ground.
According to an embodiment of the present invention, said switching means is formed of a second transistor, the control terminal of which is connected to the drain of the first input MOS transistor.
According to an embodiment of the present invention, the preamplifier includes means for setting, in static operation, the gate-source voltage of the first MOS transistor to a predetermined value.
According to an embodiment of the present invention, said means is formed of a second current source, connected between the drain of the first MOS transistor and the ground, this second source being connected as a current mirror on a third current source, the current of which is determined by a third MOS transistor with a constant gate-source voltage.
According to an embodiment of the present invention, said second transistor is an NPN-type transistor as well as transistors constitutive of the second and third current sources, the first current source being formed of a PNP-type bipolar transistor.
According to an embodiment of the present invention, the currents of the first and second sources are linked to each other via a current mirror assembly.
According to an embodiment of the present invention, the preamplifier includes a stabilization capacitor between the source and the drain of the first MOS transistor.
According to an embodiment of the present invention, preamplifier is applied to a circuit for processing a video signal analog processing.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.